Replace hidden flag with post state.

Store post state as text so Go owns the allowed values and future states such as locked remain representable without a database enum migration.

internal/store/sqlc/models.go

@@ -25,7 +25,7 @@ type Post struct {
 	Body      string
 	City      string
 	PostDate  string
-	Hidden    int32
+	PostState string
 	CreatedAt string
 	UpdatedAt string
 }